Project Summary Mitochondrial diseases are heterogeneous genetic disorders caused by the impairment of the oxidative phosphorylation (OXPHOS) system, affecting tissues that are heavily energy dependent, and often manifesting with neuromuscular symptoms accompanied by a variety of additional clinical features. Although the energetic defects arising from genetic errors in mitochondrial and nuclear DNA are often known, many aspects of mitochondrial disease pathogenesis are yet to be elucidated. As a consequence, because of the lack of defined metabolic targets, no proven effective treatments or cures are available. Our recently published studies indicate that a dramatic metabolic remodeling occurs in the skeletal muscle of a mouse model of mitochondrial myopathy. We found that a starvation-like response promotes muscle protein breakdown and amino acid catabolism to support a compensatory energy-generating oxidative flux. In this flux, glutamate is oxidized through the TCA cycle and allows for OXPHOS- independent substrate-level ADP phosphorylation. However, this compensatory process results in muscle wasting and lipids accumulation. We hypothesize that in addition to ATP synthesis impairment, OXPHOS-defective tissues must face a number of dysmetabolic problems caused by altered pathways of the intermediary metabolism. Importantly, in preliminary studies leading to this application, we have discovered that skeletal muscle from human patients with mitochondrial myopathy show similar compensatory metabolic responses. Our findings suggest that this metabolic shift towards preferred utilization of amino acids over lipids for energetic purposes underlies maladaptive effects, contributing to disease pathogenesis. In aim 1 of this application, we will investigate in depth the mechanisms and roles of the metabolic rewiring in OXPHOS-defective muscle of a mouse model of mitochondrial myopathy. In aim 2 we will investigate the muscle metabolic remodeling in human mitochondrial myopathy. In aim 3 we will test metabolic supplementation therapy in the mitochondrial myopathy mouse. At the conclusion of this study we will have improved our knowledge on the pathogenic mechanisms of mitochondrial diseases, established a functional biomarker panel for human mitochondrial myopathy, and assessed if metabolic supplementation can improve the myopathic phenotype.